iPhone 16 Pro getting warm during video calls on WhatsApp, IMO, and similar apps

The use of video will cause the iPhone processor to work harder. This would be especially true if using cellular. Harder work with the processor is going to generate more heat. It will be more if you are using cellular and the signal is not that strong. The phone getting warmer is not an issue unless it gets so hot it shuts itself down.

WhatsApp isn't owned by Apple. It's owned by Meta and they have some mighty buggy apps. But honestly, video calls make your phones battery work harder and when batteries work harder, they naturally generate heat. There really isn't much you can do about heat in some situations.